Computer systems include both disk storage devices and tape storage devices. The tape device is used for highspeed, high-volume, saving and restoring of the data files of the computer users. The disk drives are used as on-line storage for operating systems, application programs, user data files, and other processing information. A streaming tape device is a low cost, medium performance tape drive where the tape is transported directly from reel to reel with tension and velocity electronically controlled. Streaming tape drives have two modes of operation; the first mode being a start/stop and the second mode being a streaming mode. Because of the high inertia of the reels, the tape must be driven in start/stop mode in order to stop the tape in the interblock gap. The streaming mode is maintained by insuring that the tape drive commands and data (labels and data blocks) are available to the tape drive when required. The penalty for not reinstructing the tape drive in the required gap time, is that hitch back (tape respositioning) will occur. This is a very time consuming operation, and the streaming mode throughput will be adversely affected. In the streaming mode (100 inches per second in one example), the tape is moving past the read/write head at a much higher rate of speed. At this speed the tape cannot be stopped in the interblock gap because of the high inertia of the reels. After a data block has been written (or read) in a streaming mode, the tape drive assumes the next command and the data will be available while the interblock gap moves past the read/write head. Streaming mode is maintained by insuring that tape drive commands and data (labels and data blocks) are available to the tape drive head when required.
A computer user periodically saves the disk files for many purposes. In the example where the data on-line is lost, such as due to a machine malfunction or catastrophe, the data files from the last save operation can be restored. Typically, the data files are saved daily. To optimize the save function, the user needs the flexibility of saving data by file name (selective save). This minimizes the amount of output media required and reduces the time required to perform the function. This technique has a processing burden since it must locate the individual files and extract the file characteristics, physical location, length, content, etc. (prior to outputting the file). This processing commonly contributes to an increasingly significant overhead, as the average file size decreases and adversely effects the ability to keep a tape device streaming across file boundaries. An approach commonly used to save data files is to do an image dump where the entire contents of the disk drive is saved. This approach removes the processing burden, but requires more output media, a longer duration to perform a dump, and does not provide the flexibility of saving selected files.
The present invention provides a tape device to operate in a streaming mode across file boundaries by preprocessing the next file characteristics while outputting a present file. This is accomplished without requiring the system to be in a dedicated mode. The present invention also supports high speed file transfer from device to device, and particularly, transfers from disk to tape by insuring that the commands and data required are available for reinstructing a streaming tape device.